Pulsed light source for removing biological tissue

ABSTRACT

A pulsed light source for cutting away biological tissue has a control unit that controls the light source in such a way that the light source supplies a series of pulses of predetermined duration and radiation intensity. The control unit may be operated to cause the light source to supply short pulses ( 10 ) at a predetermined and/or controllable rate of repetition with a radiation intensity sufficient to cut away the tissues, and radiation followed by light emission ( 11 ) or the like with a radiation intensity that is not sufficient to cut away the tissues but is sufficient to generate heat.

[0001] The invention relates to a pulsed light source of the type statedin the preamble of claim 1 for removing biological tissue.

[0002] It is known from many medical applications that tissue can beremoved or cut with the aid of adequately intensive light, in particularlaser radiation. Removal is accompanied by heating of the surroundingtissue. The extent of this heating is determined in particular by thewavelength of the radiation used, or the coefficient of absorption ofthe tissue dependent on the latter, and by the irradiation intensity. Inthe case of high absorption in the tissue and low irradiation intensity,as is the case for example with a CO₂ continuous-wave laser, tissue ispyrolytically vaporized with a relatively great thermal side effect. Ina typical case, the crater or cut formed in soft tissue is surrounded bya carbonization layer, a zone broken up by vacuoles, a coagulation zoneand a reversibly thermally damaged region. The coagulation of the tissueproduced by the heating, and the accompanying hemostasis is of practicaladvantage in many cases, because it makes possible cuts which do notbleed. On the other hand, for applications in which as little damage aspossible to the remaining tissue and good healing of the wound areimportant, great thermal effects are disadvantageous. Carbonization ofthe tissue surface, as occurs when cutting with continuous-wave lasers,is likewise unfavorable. It has already been attempted in the case ofsuch lasers to reduce the thermal damage by increasing the irradiationintensity while at the same time shortening the time period in which itacts.

[0003] On the other hand, research in recent years has shown that, withpulsed light sources of high power and a wavelength in the ultravioletor infrared range, for example TEA-CO₂, Er:YAG, Er:YSGG or excimerlasers, hard or soft tissue can be removed without carbonization andwith only little thermal damage by a very effective thermomechanicalablation process. For instance, in the case of soft tissue, the marginaledge which coagulated after use of the free-running Er:YAG laser in vivois only about 30-40 μm. This is of particular interest for the treatmentof superficial skin lesions or for cosmetic surgery, because damage ofthe tissue beyond that which is removed is largely avoided. If, however,the capillary layer of the tissue is reached, the removal is stopped byemerging blood.

[0004] In all the surgical applications of light sources used thus far,the removal properties and the thermal side effects are coupled in asmuch as precise removal with high removal efficiency is alwaysaccompanied by a small thermal side effect, and vice versa. A knownpossible way of achieving different thermal side effects is that ofcombining a Plurality of light sources of different wavelengths in onedevice. However, the parallel operation of the two light sourcesrequired for simultaneous cutting and coagulating requires highexpenditure on apparatus.

[0005] DE 39 34 646 A1 discloses a method and an apparatus of the typestated at the beginning in which a specifically directed vaporizationwithout partial decomposition or burning is to be achieved by theluminous effect occurring in the pyrolysis process being used as acontrol signal for the control unit. By this means, the control unit iscontrolled in such a way that either the laser power, the clock ratio orthe pulse energy are changed.

[0006] Furthermore, DE 32 33 671 A1 discloses a laser apparatus having amemory means for storing a multiplicity of data records, which specifyparameters for the operating conditions for a particular laserradiation. Details on the individual parameters, and consequently anoptimum removal of tissue, are not specified here however.

[0007] The invention is based on the object of providing a pulsed lightsource of the type stated at the beginning which makes it possible withonly a single wavelength to remove tissue precisely and at the same timewith little thermal side effects and also without carbonization of thesurface and, independently of the removal, to heat the tissue in aspecifically directed and controllable manner, in order for example toproduce a coagulation zone specific for the intended use.

[0008] This object is achieved by the features specified in claim

[0009] Advantageous refinements and developments of the invention emergefrom the subclaims.

[0010] With the pulsed light source according to the invention,biological tissue can be removed precisely and with little thermal sideeffects and in addition can be heated to a variable extent.

[0011] The light emission of the light source according to the inventionis modulated in a controllable manner in such a way that, within a pulsecycle, a high-power radiation pulse used for tissue removal is followedin a defined time by a light radiation with reduced irradiationintensity, which may have the form of a reduced-power end portion of theradiation pulse or the form of a series of pulses comprising one or moreradiation pulses of which the power or energy content is not sufficientfor tissue removal and consequently leads only to the tissue beingheated.

[0012] The pulsed light source according to the invention allows ahitherto unknown range of surgical applications in which a singleradiation source can be used with little expenditure on apparatus,ranging from precision surgery with little thermal damage in areas whichare not perfused with blood, or only to a slight extent, through to theremoval of blood-perfused tissue with hemostasis. Since the thermal sideeffect, and consequently the thickness of the coagulation zone, can beadapted to the individual intervention, in any case a just sufficientand at the same time minimally damaging thermal necrosis zone can beproduced. In addition, such an enlarged coagulation zone does not leadto any sacrifices in the quality of the cut.

[0013] The light source according to the invention is preferably anultraviolet or infrared light source.

[0014] The first radiation pulse of the pulse cycle corresponds to theradiation emission as commonly used for the ablation of biologicaltissue with little damage. The tissue removal commences when a specificamount of energy per element of volume H_(abl), dependent on the type oftissue and on the irradiation intensity, has accumulated on its surface.This corresponds to a threshold value of the irradiation (F_(S)), whichis likewise dependent on the tissue and the irradiation parameters. Partof the irradiated energy remains in the tissue at the end of the pulse,heats the marginal region of the craters or cuts and leads to thethermal side effects described, in particular the coagulation of thetissue.

[0015] According to a preferred refinement of the invention, theparameters of the first radiation pulse of the series of pulses ischosen such that the heating and tissue damage produced in connectionwith the tissue removal is low. This is achieved by the combination ofhigh irradiation intensity and high absorption in the tissue (typicalcoefficient of absorption greater than 10 cm⁻¹).

[0016] The light source used for this is preferably a pulsed Er:YAG,Er:YSGG, Ho:YAG, Tm:YAG, CO, CO₂ or excimer laser.

[0017] Some data on this can be taken from the publication by R. Hibstand R. Kaufmann, ‘Vergleich verschiedener Mittelinfrarot-Laser für dieAblation der Haut’ [Comparison of various mid-infrared lasers forablation of the skin], Lasermedizin [Laser Medicine], Vol. 11 (1995),pages 19-26.

[0018] Typical values for the Er:YAG laser are:

[0019] energy required for removal per element of volume H_(abl)=1.5kJcm⁻³

[0020] threshold value of the irradiation intensity about 1 Jcm⁻²

[0021] pulse duration in the range from 150 to 600 μs

[0022] irradiation in clinical use on the skin about 5-20 Jcm⁻²

[0023] average irradiation intensity several 10 kWcm⁻²

[0024] if removal is over a surface area, size of the spot 1 to 3 mm indiameter

[0025] The laser-related power is calculated from the irradiationintensity and the size of the spot.

[0026] The coagulation zone caused by the removing pulse is enlargedaccording to the invention by emitting after the short pulse leading toremoval a respectively following light radiation with an irradiationintensity and/or irradiation which is not sufficient for the removal oftissue but produces a thermal effect.

[0027] This subsequent light radiation in the form of a pulse endportion of reduced irradiation intensity or of at least one, butpreferably more than one light pulses is dimensioned with regard to itspower or energy such that, given a predetermined size of the irradiationzone, the removal threshold value of the tissue is not reached.

[0028] According to a refinement of the invention, at least one pulsewith low irradiation intensity is used for this. In order that thetissue is not removed and is only heated, pulses with such a lowirradiation intensity that, as a result of the heat conduction, theenergy per element of volume accumulated at the surface remains belowH_(abl), i.e. the irradiation intensity remains below the thresholdvalue required for removal.

[0029] To estimate the upper limit of the irradiation intensity, it maybe assumed that the energy H occurring per element of volume resultsfrom the supply of energy produced by light absorption and an energyloss proportional to H:$\frac{H}{t} = {{\mu \cdot I_{0}} - {\frac{1}{\tau} \cdot {H.}}}$

[0030] (I₀: irradiation intensity, μ: coefficient of absorption).

[0031] The thermal relaxation time τ used as the proportionality factorfor the rate of loss can be estimated from the known formulae. Itdecreases quadratically with the heated volume, and therefore withincreasing μ. The threshold value of the irradiation intensity I_(S) isreached when, in the state of equilibrium (dH/dt=0) , the energy densityat the surface is equal to H_(abl). It thus follows from the aboveequation that: $I_{S} = \frac{H_{abl}}{\mu \cdot \tau}$

[0032] For the Er:YAG laser, the thermal relaxation time of the issuecan be estimated as a few μs for the beginning of the irradiation, sothat the remaining values (see above) give an irradiation intensityI_(S) in the kWcm⁻² range. With increasing enlargement of the heatedregion, I_(S) then decreases. The exact progression is difficult tocalculate here. For a layer of a thickness of, for example, 80 μm, thethermal relaxation time is about 30 ms, which leads to a maximumpermissible irradiation intensity of about 5 Wcm⁻². An advantageousrefinement of this alternative is therefore a progression withdecreasing irradiation intensity. The irradiation intensity (power) andthe duration of the pulse then determines the extent of heating.

[0033] If the required difference in the irradiation intensity betweenthe removing pulses and the heating pulses is technically difficult toaccomplish in the case of a given laser, according to a furtherrefinement of the invention it is envisaged to use a sequence of pulseswith an energy content below the removal threshold value.

[0034] As a point of reference for this threshold value, the thresholdvalues F_(S) (see above) determined from the removal measurements may beused. The threshold values increase with decreasing irradiationintensity (they are theoretically infinite at an irradiation intensityof I_(S)) and decrease in the case of preheated tissue. Thus, for theEr:YAG laser, initially F_(S)=1 Jcm⁻² would be assumed and, in anexperimental situation, the irradiation intensity of each individualpulse or its duration would be changed such that removal no longer quitetakes place. The individual factors, irradiation intensity and pulseduration, are governed by the technical requirements of the lightsource; primarily decisive for the effect is their product.

[0035] According to one embodiment, the irradiation intensity and theduration of the pulses following the first radiation pulse may vary fromone another. This is appropriate, for example, for the Er:YAG laser if,for supplying the pumping flashlamp of this laser, the energy of asingle capacitor bank is used for generating the entire series ofpulses. The decreasing voltage causes the laser pulses to beincreasingly weak, which however can be compensated by a correspondinglyprolonged pulse duration.

[0036] The optimum time interval between the subpulses or between theremoving pulse and the series of pulses for heating results from thethermal relaxation time of the tissue surface. To be able to introduceas much energy as possible into the tissue without removing it, it isfavorable to allow the tissue surface to cool between two such pulseswith respect to the temperature leading to removal. In order at the sametime to produce a great depth of the coagulation, this cooling shouldnot proceed right down to the (physiological) starting temperature(typically 37° C). Rather, the subsequent hearing by the following pulseshould take place at the latest when the surface has reached thetemperature required for the desired coagulation, of about 60° C. to 70°C. This time period increases with the optical depth of penetration ofthe radiation used.

[0037] In addition, the cooling behavior of the surface depends on itsprehistory. In the case of the first laser pulse, the superficialheating of the tissue leads to a very steep temperature gradient with acorrespondingly rapid falling of the temperature, caused by the heatconduction. The heat conduction also causes layers of tissue below thesurface to be heated, so that the temperature gradient for a subsequentheating pulse is smaller. The increase in she thermal relaxation timewith the number of heating pulses can be seen from a measurement of thesurface temperature. An optimized sequence of heating pulses willtherefore generally have different time intervals between the individualpulses. By analogy, the energy content of the individual pulses will bedifferent.

[0038] Model calculations and measurements show for the Er:YAG laserthat the temperature increase required for a coagulation of the skin invivo from 30 K to 40 K is reached again at the surface a few ms afterthe end of the pulse. For the Ho:YAG laser, about 20 times this valuecan be expected. The exact times are to be determined experimentally ineach case for the tissue under consideration and the wavelength used.

[0039] For effects other than coagulation, for example hyperthermia,other temperatures and times which can be readily determined by a personskilled in the art are of course critical.

[0040] In the case of this embodiment of the series of pulses used forheating, the energy introduced altogether (per element of surface) intothe tissue, and consequently the depth of coagulation, can beadvantageously controlled by the number of pulses in the series ofpulses following the first pulse.

[0041] In the case of a modified embodiment, as an alternative to thepredetermination of fixed parameters for the individual heating pulses,a control of the pulse energy levels, durations and interpulse periodson the basis of the surface temperature measured continuously orintermittently can be used. As soon as the surface temperature dropsbelow a predetermined minimum value (for example 70° C.), the laser isactivated. The laser emission is stopped again when the preset upperlimit value (for example 200° C.) is reached.

[0042] In the case of this way of accomplishing the series of pulsesused for heating, the energy introduced altogether (per element ofsurface) into the tissue, and consequently the depth of coagulation, canbe advantageously controlled by the number of pulses in the series ofpulses following the first pulse.

[0043] Of course, the optimized series of heating pulses may also beused without the removing pulse for coagulating. Similarly, it may beadvantageous to apply the heating pulses before the removing pulse (aswell), if, for example, infected tissue is to be killed off before theremoval, which is accompanied by a dispersion of tissue fragments.

[0044] The invention is explained in more detail below with reference tothe drawings, in which:

[0045]FIG. 1 shows a section through a region of tissue afterirradiation in the case of high tissue absorption and low irradiationintensity,

[0046]FIG. 2 shows a section through a region of tissue afterirradiation in the case of high tissue absorption and high irradiationintensity,

[0047]FIG. 3 shows a section through a region of tissue afterirradiation with a pulsed light source according to the invention,

[0048]FIG. 4 shows a first embodiment of a pulse which has abeginning-of-pulse portion, inducing removal, and an end-of-pulseportion, following the first, with reduced irradiation intensity,

[0049]FIG. 5 shows a preferred embodiment of a sequence of pulses with afirst pulse, inducing removal, and a pulse following the latter withdecreasing irradiation intensity,

[0050]FIG. 6 shows a further embodiment of a sequence of pulses with afirst pulse, inducing removal, and a series of pulses following thelatter with increasingly weaker, but correspondingly prolonged pulses,

[0051]FIG. 7 shows a configuration of an apparatus with a control of thelaser in dependence on the surface temperature of the tissue.

[0052] In FIG. 1 there is shown a section through a region of tissue asobtained, for example, in the case of high absorption in the tissue andlow irradiation intensity. This occurs, for example, in the case of theCO₂ continuous-wave laser, which is directed at the tissue surface 1.The crater or cut 2 formed in the tissue is surrounded by acarbonization zone 3, a zone 4 broken up by vacuoles, a coagulation zone5 and a reversibly thermally damaged region 6. The coagulation of thetissue produced by the heating and the accompanying hemostasis is ofpractical advantage in many cases, because it makes possible cuts whichdo not bleed. For applications in which as little damage as possible tothe remaining tissue and good healing of the wound are important, greatthermal effects are disadvantageous. Carbonization of the tissue surfaceis likewise unfavorable.

[0053] In FIG. 2 there is shown a section corresponding to FIG. 1, whichshows the irradiation of a pulsed light source of high power and awavelength in the ultraviolet or infrared range. Examples of such alight source are TEA-CO₂, Er:YAG, Er:YSGG or excimer lasers. With theselasers, hard or soft tissue can be removed without carbonization andwith only little thermal damage by a very effective thermomechanicalablation process. The zone 5 which, in the case of soft tissue,coagulated after use of the free-running Er:YAG laser has in vivo only athickness of about 30-40 μm. This is of particular interest for thetreatment of superficial skin lesions or for cosmetic surgery, becausedamage of the tissue beyond that which is removed is largely avoided.If, however, the capillary layer is reached, the removal is stopped byemerging blood.

[0054]FIG. 3 shows a section corresponding to FIGS. 1 and 2 through atissue after irradiation with the pulsed light source according to theinvention. As will be explained in more detail below with reference toFIGS. 4 and 5, in this case the light emission of a pulsed ultravioletor infrared light source is modulated in a controllable manner in such away that, within a pulse cycle, a high-power pulse sufficient for tissueremoval is followed in a defined time by a series of pulses comprisingone or more pulses of which the power or energy content is notsufficient for tissue removal and consequently leads only to the tissuebeing heated. In this case, the crater 2 is surrounded by a coagulationzone 5 of controllable size. In this way, it is possible to removetissue precisely and at the same time with little thermal side effectsand without carbonization of the surface and, independently of theremoval, can be heated in a specifically directed and controllablemanner.

[0055]FIG. 4 shows a first embodiment of a pulse for achieving theremoval shown in FIG. 3. In this case, each pulse comprises a short,first beginning-of-pulse portion 10, sufficient for removal, and asubsequent end-of-pulse portion of reduced irradiation intensity. Withregard to the individual parameters of the pulse portions, reference ismade to the discussion above.

[0056]FIG. 5 shows a second embodiment of a sequence of pulses forachieving the removal shown in FIG. 3. In this case, in a pulse cycle, afirst, short pulse 10, sufficient for removal, is followed by at leastone further pulse 11, which is separated from the pulse 10 by a timeinterval, has an irradiation intensity decreasing over time and merelyproduces a heating effect.

[0057] In FIG. 6 there is shown a further embodiment of a sequence ofpulses in which, in a pulse cycle, the short pulse 10 of highirradiation intensity, sufficient for removal, is followed by a sequenceof pulses 12 to 14 of which the irradiation intensity decreases in eachcase, but the duration of which increases.

[0058] Of course, the irradiation intensity of the pulses 11, or 12 to14, following the first pulse 10 and their duration could also beconstant, as long as they do not lead to further damage or removal ofthe tissue. Furthermore, the number of these pulses 12 to 14 may beselected according to the purpose in question on the basis of thecriteria stated at the beginning for the respective application.

[0059] An alternative to the predetermination of fixed parameters forthe individual heating pulses is the control of the pulse energy levels,durations and interpulse periods on the basis of the surface temperaturemeasured continuously or intermittently, for example between theindividual pulses. As soon as the surface temperature drops below apredetermined minimum value (for example 70°C.), the laser is activated.The laser emission is stopped again when the preset upper limit value(for example 200°C.) is reached.

[0060] A possible refinement of such an apparatus in the form of ahand-held appliance is diagrammatically shown in FIG. 7. The laserradiation from a laser source Q is deflected by means of abeam-splitting mirror S, which is transmissive to thermal radiation, andis focussed on the tissue by a lens L which is transparent to laserradiation and thermal radiation. The irradiated surface region of thetissue is likewise projected through the lens L onto the end face of alight-conducting fiber transmitting the thermal radiation (for example asilver halide fiber or chalcogenide fiber). This fiber conducts thethermal radiation to an infrared detector D. From the output signal ofthe latter, which is amplified in an amplifier V, after appropriatecalibration the surface temperature of the tissue being worked at thetime can be calculated, and this can then be used for the describedcontrol of the laser.

[0061] In the case of this embodiment of the series of pulses used forheating, the energy introduced into the tissue altogether (per elementof surface), and consequently the depth of coagulation, can beadvantageously controlled by the number of pulses in the series ofpulses following the first pulse.

[0062] Although only laser light sources have been mentioned above asexamples of the light source, these examples are in no way restrictive,since other light sources, the light generating process of which is notbased on the laser principle, with a corresponding wavelength andirradiation intensity may also be used, such as for example pulsedhigh-pressure gas discharge lamps with xenon or other gas filling.

1. Pulsed light source for removing biological tissue, having a controlunit for controlling the light source in such a way that it supplies asequence of pulses in each case with a predetermined duration andirradiation intensity, characterized in that the control unit can beoperated in such a way that the light source supplies at a predeterminedand/or controllable repetition rate short pulses (10) with anirradiation intensity and irradiation sufficient for removing tissue anda respectively subsequent light radiation (10 b; 11; 12-14) with anirradiation intensity and/or irradiation which is not sufficient forremoving tissue, but produces a thermal effect.
 2. Light sourceaccording to claim 1 , characterized in that the light source suppliesat the repetition rate pulses which in each case comprise a first pulseportion (10 a) of short duration with an irradiation intensity andirradiation sufficient for removing tissue and a subsequent pulseportion (10 b) of which the irradiation intensity is modulated in such away that, the removal is stopped after the first pulse portion (10 a)and the tissue is only heated during the second pulse portion (10 b). 3.Light source according to claim 1 , characterized in that the lightsource supplies at the repetition rate sequences of pulses whichcomprise at least one short first pulse (10) with an irradiationintensity and irradiation sufficient for removing tissue and at leastone further pulse (11; 12-14) with an irradiation intensity and/orirradiation which is not sufficient for removing tissue, but leads to athermal effect.
 4. Light source according to one of claims 1 to 3 ,characterized in that the short pulses (10) or pulse portions (10 a)produce a removal of tissue with little damage and only littlecoagulation of the marginal regions of the removal.
 5. Light sourceaccording to one of claims 1 to 4 , characterized in that the shortpulses (10) or pulse portions (10 a) have a high irradiation intensityand the emitted light has a high absorption in the tissue.
 6. Lightsource according to one of the preceding claims, characterized in thatthe light source through is a laser emitting ultraviolet or infraredlight.
 7. Light source according to claim 6 , characterized in that thelaser is a pulsed erbium, holmium, thulium, CO₂ or excimer laser. 8.Light source according to one of the preceding claims, characterized inthat the irradiation applied to the tissue surface with each short pulse(10) or pulse portion (10 a) lies in the range from 1 to 250 Jcm⁻² perpulse or pulse portion.
 9. Light source according to one of thepreceding claims, characterized in that each short pulse (10) or pulseportion (10 a) has a power of more than 500 watts and a duration of 50to 1000 microseconds.
 10. Light source according to one of the precedingclaims, characterized in that the energy of the further pulses (11;12-14) or of the subsequent pulse portion (10 b) is in each casedimensioned in such a way that, given a predetermined size of theirradiation zone, the removal threshold of the tissue is not reached.11. Light source according to one of the preceding claims, characterizedin that the at least one further pulse (11) has an irradiation intensitydecreasing with time.
 12. Light source according to one of claims 1, 2and 4 to 10, characterized in that the subsequent pulse portion (10 b)initially has an irradiation intensity increasing with time and thensubsequently decreasing.
 13. Light source according to one of claims 1to 11 , characterized in that the individual successive further pulses(12-14) of a sequence of pulses in each case have a decreasingirradiation intensity, but an increasing pulse duration and in that,given a predetermined size of the irradiation zone, the energy contentof each and every one of these pulses lies below the removal thresholdvalue.
 14. Light source according to one of claims 3 to 11 and 13 ,characterized in that each sequence of pulses has a predeterminedduration.
 15. Light source according to one of the preceding claims,characterized in that the energy content of the pulse (11) following theshort pulse (10) or pulse portion (10 a), or of the following series ofpulses (12-14) or of the subsequent pulse portion (10 b), is sufficientfor coagulation of the tissue.
 16. Light source according to one of thepreceding claims, characterized in that the control unit controls theenergy content of the pulse (11) following the short pulse (10) or pulseportion (10 a), or of the following series of pulses (12-14) or of thesubsequent pulse portion (10 b) and/or the intervals between successivepulses in dependence on a measurement of the surface temperature of thetissue.